When I was first asked by John Fernando to be the RECORDER special editor for the theme: Seismic Inversion / Time-lapse / Rock Physics, my first thought was that I was being asked to put together the “theory of everything” in exploration seismology. Come on, John, what topics have you actually left out? However, having assembled the articles for this issue, one in each of the named topics, I now realize that these three areas constitute the backbone of a new direction that exploration seismology has been moving over the last twenty years. The title of this introduction, “Quantitative Seismic Interpretation”, was shamelessly “stolen” from a book of the same name by Avseth, Mukerji, and Mavko (Cambridge University Press, 2005). In fact, the sub-heading of that book, “Applying Rock Physics Tools to Reduce Interpretation Risk”, would also be an excellent title for this special issue. In their textbook, Avseth et al. start with three chapters on rock physics. If you will pardon the pun, rock physics really is the bedrock for everything we do in quantitative seismic interpretation. To understand the rocks (and the fluids) is to understand the seismic.
The first article in this special issue, entitled “A Gassmannconsistent rock physics template”, was inspired by all three of the authors of the above textbook. In it, I extend the rock physics template, or RPT, that was developed by Avseth (and Ødegaard) using the work of Mukerji and Mavko in their published papers (see the paper for the specific references). As can be seen in this paper, an RPT is an excellent way of understanding the way in which seismically derived parameters such as VP/VS ratio and P-impedance can be related to rock properties such as porosity, fluid content, pressure and shale content. The specific RPT that I develop uses Gassmann theory not only for the fluid substitution, but also for the porosity change.
But how do we estimate VP/VS ratio and P-impedance from the seismic data? The answer to this question allows me to transition to the second of the papers in this special issue:
“Joint simultaneous inversion of PP and PS angle gathers”, by Hampson and Russell. In this paper, the authors first discuss how to simultaneously estimate P-impedance and Simpedance (and optionally density) from pre-stack PP seismic data. Note that this also allows us to estimate VP/VS ratio, which is equivalent to the ratio of P-impedance to Simpedance. The authors then show how to extend this theory to a joint inversion involving both PP and PS data. Examples are shown from both synthetic seismic data derived from well logs, and real seismic data.
Finally, the paper by Riazi and Lines, “Reservoir Characterization using time-lapse seismology over a cold production heavy oil reservoir”, utilizes rock physics and inversion as well as time-lapse seismology to identify areas of production in a heavy oil reservoir. This paper therefore draws together all the threads discussed in this introduction. The authors first discuss the basics of time-lapse seismology and then proceed to show its applicability to the Primate heavy oilfield near the border of Alberta and Saskatchewan. They first discuss how to process the repeated 3D seismic surveys in order to preserve production related amplitude and time changes and remove non-production effects. They then perform fluid replacement analysis to predict the expected production changes. Finally, they perform seismic inversion to validate the rock physics predictions.
In summary, the three papers in this special section all discuss parts of an integrated approach to quantitative seismic interpretation that first involves the use of rock physics and fluid replacement modelling to predict the seismic response from the rocks and fluids, and then confirms this response by inverting either the pre- or post-stack data. My expectation is that this area of seismic interpretation will continue to grow and that it is important for geoscientists to have a good grasp of these techniques. However, as always, the best interpretations still come from the creativity of the individual doing the interpretation.